Yet again, I am have problem with the curve-modifier. So I did exactly (or at least I believe I did) as what others told me in my last question:
set origin of the curve to its geometry > set cursor to the curve > set origin of the tube to its geometry > move the tube to the cursor
And then I applied the curve-modifier and move it along the Z-axis. But for some reason, the tube ran out of the curve and they were not aligned any more. The same happened to the cylinder array that is supposed to run along the curve as well. I tried rotating the tube and change the axis, but it still didn't work.
Could anyone please point out what I missed in these steps? The file is attached as follows: the tube and cylin_array are the application of curve modifier, whereas the tube.001 and cylin_array.001 are after the application.